The present invention relates to by-pass flowmeters and more particularly to by-pass flowmeters incorporating a rotameter.
In a rotameter the stream of fluid to be measured is made to pass through a constriction, the constriction being arranged so that its size is varied to accommodate the flow while the differential head or pressure drop is held constant. The variation of the opening (that is, the degree of constriction) is automatically brought about by the motion of a weighted piston or float supported by the fluid. More particularly, the rotameter consists essentially of a plummet or metering float which is free to move in a vertical, slightly tapered tube with the small end down. The fluid enters the lower end of the tube and causes the plummet to rise until the annular area between the plummet and the wall of the tube is such that the pressure drop across this constriction is just sufficient to support the plummet. Typically, the tapered tube is of glass and carries a linear scale thereon through which the position of the floating plummet is correlated with the flow rate.
Where the volume of flow to be measured is large, however, it is customary to use a by-pass flowmeter in order to keep the size of the rotameter itself within practical limits. In such a by-pass flowmeter incorporating a rotameter, the fluid flow through the flowmeter is divided into two streams, one through a tube containing the rotameter and the other through a chamber able to accommodate a much larger flow than the rotameter. By means of inlet and outlet orifices to both paths, a small volume of the total flow passes through the rotameter while the remaining larger volume passes through the chamber. If the two flows are in parallel, the pressure drop across each is the same as the total pressure drop across the flowmeter.
To date, by-pass flowmeters have positioned the rotameter or other measuring device not in the center of the total flow through the flowmeter, but rather to one side thereof. Frequently the rotameter has been placed in a by-pass chamber outside of the main flow (but connected thereto by suitable tubing) so that the readings of the rotameter may be made easily visible through a transparent wall of the by-pass chamber while the main flow chamber is constructed of opaque material. In other cases the rotameter is disposed within the main flow chamber, but against the wall thereof, presumably to facilitate reading of the rotameter. Whatever the reasons for the conventional design, the result is that either the rotameter reading is indicative of the pressure drop to one side of the center of the main flow (and this may differ from the pressure drop in the center of the main flow) or the rotameter reading reflects the pressure drop occurring in the center of the main flow, but only as that pressure drop is also affected by the twists and turns of the tubing connecting the rotameter inlet and outlet to the center of the main flow. The former situation occurs when the tubing connecting the rotameter inlet and outlet to the main flow chamber is linear, but the connections terminate at points other than the center of the main flow; the latter situation occurs when the tubing connects the rotameter inlet and outlet to the center of the main flow chamber, but the tubing is not linear. In either case, the reading of the rotameter is not truly indicative of the main portion of the flow rate. Furthermore, compensation of these inaccuracies by way of calibration is rendered exceedingly difficult, if not impossible, as the degree of inaccuracy thus introduced varies as the range of flow rates varies.
Accordingly, it is an object to provide a by-pass flowmeter of enhanced accuracy.
Another object is to provide such a flowmeter whose readings accurately reflect only the pressure drop across the central portion of the main flow.
A further object is to provide such a flowmeter which is compact, inexpensive and easy to use.